In order to increase the efficiency and power output of modern gas turbines, the combustion temperatures have been constantly raised. More recently, NOx and CO2 emissions regulations have become stricter, maintaining low emission level will thus be an incentive of increasing importance. This can be addressed by reducing the unmixed air in the combustion process. While reducing the amount of effusion cooling air downstream the fuel injection location helps reducing emissions, the cooling of the hot gas path walls remains important for ensuring the specified operation lifetime. As an alternative to conventional effusion cooling, as disclosed in US 2012/0047908 A1, highly efficient near wall cooling schemes can provide the required cooling of the burner structure.
A combustion chamber with a combustion-chamber wall of double-walled design mentioned above emerges from EP 0 669 500 B1. There is a flow of compressed combustion feed air for cooling purposes through the enclosed intermediate space of the combustion-chamber wall of double-walled design which surrounds the combustion zone, the combustion-chamber wall of double-walled design being cooled by way of convective cooling. At the same time, this approach minimizes the amount of cooling air emitted into the hot gas path; unfortunately the manufacturing of such near wall cooling systems is very difficult. One approach could be the casting of double-walled hollow core structures. However, the drawback of this manufacturing method is its high complexity resulting in a high scrap rate and thus high cost. In addition, the casting approach suffers from its inherent design limitations and the very long lead-time for any design modification. Another problem is the large size and complexity of burner arrangement especially premix burner arranged along an annular shaped front panel of an annular combustor. Precision casting of double-wall, hollow core structures is normally reserved for smaller components like turbine blades and vanes, where a high prize can be easier accepted.
Another important aspect of operational behaviour of a gas turbine concerns operation flexibility. Here, the main limitations are pulsation levels during part load or transients, which have to be carefully controlled. In gas turbines, during operation, heavy thermo acoustic pulsations, which are heavy pressure oscillations, can occur in the combustion chamber, because of an incorrect combustion of the fuel such as gas or oil. These pulsations subject the hardware of the combustion device and the turbine to heavy mechanical vibrations that can result in the damage of individual parts of the combustion device or turbine.
In order to absorb such pulsations, combustion devices are usually provided with dampers, such as the Helmholtz dampers. Helmholtz dampers consist of a resonance chamber that is connected via a damping tube to the interior of the combustion chamber or the medium surrounding the combustion chamber.
US2005/0229581 discloses a reheat combustion device that has a mixing tube followed by a combustion chamber; the mixing tube has at its front panel an acoustic screen provided with holes and, parallel to it, an impingement plate also provided with holes. The acoustic screen and the impingement plate define a chamber connected to the inner of the combustion chamber via the holes of the acoustic screen and to the outer of the combustion chamber via the holes of the impingement plate. The chamber between the impingement plate and acoustic screen defines a plurality of Helmholtz dampers such that, since a plurality of dampers are associated to each reheat combustion device, the damping effect is improved. However the air flow within the chamber between the impingement plate and the acoustic screen is not guided, the cooling efficiency is not optimised; this makes different parts of the combustion chamber to be cooled in different way and to operate at different temperatures. In addition, manufacturing is very hard.
Another approach for reducing thermo acoustic pulsation efficiently concerns the combination of acoustic damping and near wall cooling as disclosed in EP 2 295 864 A1. Here a combustion device for a gas turbine comprises a portion having a first and a second wall. A first passage connects the zone between the first and second wall to the inner of the combustion device and a second passage connects said zone between the first and second wall to the outer of the combustion device. Between the first and second wall a plurality of chambers as being Helmholtz dampers are defined, each connected with one first passage and at least one second passage.
In the production of a prototype of a gas turbine the front panel of an annular combustor operated with a multitude of burners was manufactured as one full size part. After brazing the complete front panel sandwich structure enclosing the before described Helmholtz damper chambers, the front panel was hand-welded to the body of the annular combustion chamber. The procedure has been found to be rather complicate; in addition the welding area will be exposed to very high temperatures during operation of the gas turbine, so that life expectancy of this welding joint appears rather limited. Moreover, best engineering practice and much care was used for vacuum brazing of the large front panel prototype structure. It will be very difficult to maintain this manufacturing quality level in a commercial production process with a much higher volume of parts.